1. Field of the Invention
This discovery generally relates to solvents or vehicles for compounds that target sensory elements on nerve fibers, which are usefully administered to refresh and to cool the skin and mucous membranes.
2. Description of the Related Art
About three decades ago, a group of scientists synthesized over 1200 compounds in an attempt to find cooling agents that had properties better than menthol. Their results were summarized in a paper (Watson et al. New compounds with the menthol cooling effect. J. Soc. Cosmet. Chem. 29, 185-200, 1978.). From this research, an N-alkyl-cycloalkyl- and an N-alkyl-alkyl carboxamide, WS-3 (2-Isopropyl-5-methyl-cyclohexanecarboxylic acid ethylamide), WS-5 ([(2-Isopropyl-5-methyl-cyclohexanecarbonyl)-amino]-acetic acid ethyl ester), and WS-23 (2-Isopropyl-2,3,N-trimethyl-butyramide), respectively, reached the market and are used as additives to confectionery, comestibles (e.g., candy, chewing gum), and toiletries. Other menthol-like cooling compounds in commercial use for applications to skin and mucous membranes are, for example, menthyl lactate (Frescolat ML), menthoxypropanediol (Cooling Agent 10), and 2-isopropyl-5-methylcyclohexyl 4-(dimethylamino)-4-oxobutanoate. The recent information on cooling agents used for topical applications has been reviewed (see, e.g., Erman, M. B. “Cooling agents and skin care applications”, Cosmetics & Toiletries, 120, 105-118, 2005; Erman, M. B. “Progress in physiological cooling agents”, Perfumer & Flavorist, 29, 34-50, 2004; Jacobs, P. and Johncock, W., “Some like it cool”, Parfumerie and Ksometik, 80, 26-31, 1999).
Cooling compounds are described in U.S. Pat. No. 6,919,348 (Wei et al., Jul. 19, 2005). Other molecules investigated by Wei are described in: US 2005/0059639, published Mar. 17, 2005, Ophthalmic Compositions and Methods for Treating Eye Discomfort and Pain; US 2005/0159394, published Jul. 21, 2005, Aryl-Substituted Derivatives of Cycloalkyl and Branched Chain Alkyl Carboxamides and Carboxylic Acids Useful as Antinociceptive Drugs For Peripheral Targets; US 2005/0187211, published Aug. 25, 2005, N-Aryls-Carboxamide Compositions and Methods; and WO 2006/103401, N-Alkylcarbonyl-Amino Acid Ester and N-Alkylcarbonyl-Amino Lactone Compounds and Their Use, published Oct. 5, 2006.
In the delivery of these cooling agents to the desired biological targets, formulations for the skin (e.g. lotions, creams, ointments) and formulations for the respiratory tree or oral cavity (e.g. vapors, sprays) that are liquid, semi-liquid, or non-particulate, require a solvent for the active cooling ingredient. Frequently, two or three carbon alcohols such as ethanol, isopropyl alcohol, and racemic 1,2-propanediol, are used.
Weil et al. 2005 [Molecular Pharmacology 68: 518-527, 2005] reported that 0.5% ethanol in the medium inhibited the TRP-M8 receptor response to (−)-menthol by 50%, and the response is almost totally lost at 3% concentration of ethanol. The TRP-M8 receptor is the putative target on neurons that mediate cooling and anti-irritant sensations. Benedikt et al. 2007 [J. Neurochemistry 100: 211-224, 2007], confirmed Weil's results and noted that the activity for in vitro inhibition was methanol<ethanol<isopropanol<butanol. Dimethylsulfoxide, a solvent with a dielectric constant similar to water, was claimed to be less inhibitory. Benedikt et al. discussed the possible mechanisms of ethanol interference with receptor activity and suggested: 1) low molecular weight alcohols are absorbed into lipid bilayers, and may seriously affect the mechanical properties of cell membranes and/or 2) affect secondary intracellular messengers such as phosphatidylinositol-4,5-biphosphate that transduce the receptor activation to neuronal signals. These studies by Weil et al. and by Benedidkt et al. showed that the solvent medium is important for the bioactivity of cooling agents.
Ideally, a solvent should dissolve the cooling agent and deliver the active ingredient to target without interfering with bioactivity. Chemicals such as 1,2-ethanediol, methanol, dimethylsulfoxide, and butanols are not used in topical formulations because of potential hazards. Thus, the choice of an ideal solvent among the two and three carbon alcohols is limited.
The short-chain alcohols are generally thought to interact with biological membranes by non-specific physical forces such as interfacial tension, mechanical compressibility per area/molecule, and affecting the permeability parameters of fluid lipid bilayers (Ly and Longo, Biophysical J. Biophys. J. 87: 1013-1033; 2004). Harris et al. (Ethanol's molecular targets. Science Signaling, Jul. 15, 2008), recently summarized evidence for an alternative view, namely, that ethanol acts on specific “pockets” on protein receptor surfaces to modulate function.